In order to study the vibration behavior of structures, use is made of equipment that is constituted by a test bench fitted with one or more actuators enabling reciprocating motion of controlled frequency and amplitude to be imparted to said bench. Structures for testing are fastened to the bench, oscillatory excitation generated by said actuators is transmitted to the structures via the bench, and the response of the structures to said excitation is measured using accelerometers. Equipment of that type exists in very many variations: the structures for testing may present a very wide range of masses and dimensions (electronic cards weighing a few grams to mechanical structures weighing several (metric) tonnes), and they may need to be subjected to excitation at a very wide variety of frequencies and amplitudes.
In order to perform vibration tests on structures of large dimensions (several tonnes) at relatively low oscillation frequencies (generally less than 1 kilohertz (kHz)), use can be made of hydraulic actuators, such as double-acting jacks. Like any mechanical element, a hydraulic actuator presents finite stiffness; the assembly constituted by said actuator, the test bench, and the structure thus behaves like a coupled vibratory system constituted by the actuator(s), the bench, and the load. The finite stiffness of the actuator(s) affects (disturbs) the response of the system, particularly when the load is heavy. The effects of such coupling are numerous, complex, and harmful to the quality of the testing. One of the most awkward effects concerns the “suspension” mode of the system that is constituted by the load mounted on the actuator(s). From the point of view of vibration measurement, this mode does not correspond to dynamic behavior of the load in vibration, but to “parasitic” dynamic behavior coupling the load and the test installation. Furthermore, this very strong dynamic behavior makes it considerably more difficult to control excitation of the load. Depending on the mass of the load, the number of hydraulic actuators, and their respective stiffnesses, this resonant frequency sometimes lies within the frequency band of the test; this leads to very strong undesired coupling with the resonant modes of vibration of the structure, thereby disturbing measurement of its vibratory behavior. Furthermore, from the point of view of controlling the excitation, the lower the frequency of the suspension modes, the greater their amplitude, and the greater the difficulty for the installation in performing the specified tests, since that requires the installation to eliminate or greatly reduce the parasitic dynamic behavior.